Adsorption performance and mechanisms of MgO-modified palygorskite/biochar composite for aqueous Cd (II): Experiments and theoretical calculation

The treatment of Cd2+ pollution in water has been a hot topic of research, while the studies for the preparation and application of biochar composites to treat Cd2+ and the corresponding micro-mechanisms were still lacking. In this study, a novel MgO-modified palygorskite/biochar derived from rice straw composite (MPBC) was synthesized to remove Cd2+ in the aqueous solution. Batch experiments were conducted to evaluate the adsorption ability for Cd2+ on MPBC. The results showed that the maximum adsorption capacity of MPBC was 441.1 mg/g, which was 4.6 times higher than that of the original biochar (BC). Adsorption mechanisms were clarified combine systematic characterization and density functional theory (DFT) calculations. Precipitation, complexation, ion exchange, and Cd-π interaction were the main adsorption mechanisms, and the contribution of the ion exchange, compared with 12.08% of BC, increased to 37.82%. The O atom of the MgO group performed negative electrostatic potentials as an electrophilic active site. The nucleophilic Cd tends to transfer electrons to the electrophilic O atom of the MgO group. Furthermore, the density of state (PDOS) identified that hybrid overlap occurred between MgO and Cd, and a new chemical bond was formed in the adsorption process. Overall, this work offers a new approach to removing aqueous Cd2+ and uses biowaste to create a sustainable environment.